Dynaco 410 tests

In the 41 years I've owned it, the Dynaco Stereo 410 amplifier has been perfectly reliable.  I had it refurbed once in 2001 because I was thinking it caused hum, but in fact the hum was being caused by my analog crossover, as I discovered afterwards.  The audio shop in Austin replaced all the small electrolytic capacitors on the two amplifier circuit boards.  They did not change the very large 22,000 uF Sangamo Computer Grade capacitors saying that it was not necessary nor cost effective.  I used the amplifier for only about 2 years after the refurb.

I bought this amplifier in 1982 or so for use with subwoofers.  I used two McIntosh ML2's for subwoofers, powered by this amp.  For this use I did not need the fan, so I disconnected it, and instead drilled holes in the back of the case for ventilation.  This works ok so long as you don't need much power.  I have reconnected the fan for test and sale purposes.  It is fairly quiet at the low speed.

I also covered the top of the chassis with duct tape.  This does somewhat reduce vibration either with or without the fan running, but hockey tape would have been better.

The amplifier switch is broken and the amplifier is always on when plugged in.  It should be switched with an external 15A power strip.   Inside the amp there is a line fuse and thermal cutouts mounted near the output transistors.  There are also rail fuses.  There are externally accessible speaker fuses but they have been bypassed and the fuse caps are missing.  There is a sensor that kicks the fan into high speed if it gets too hot.

Other than the broken power switch, the amplifier works like new.

At full power, 200W into 8 ohms (40V RMS):

Rated at 0.25% distortion at any frequency 20-20kHz

Right channel THD + N

1000 Hz    0.014%

20 Hz        0.014%

20 kHz        0.35%

Left Channel THD + N

1000 Hz    0.03%

20 Hz        0.024%

20 kHz        0.3%

S/N, A weighted

Relative to Rated Power

L: 119.5 dB

R: 110.5 dB

Relative to 1 watt

L: 97.0dB

R: 88.0 dB

S/N relative to rated power is so good it's hard to believe.  In case I miscalculated, the actual measurements were 120uV for the right channel and 42uV for the left channel, read from my A weighted Meguro meter.

Overall this is much better than I had expected for an amplifier of this epoch and age.  This was a stripped down version of the Dynaco 400 designed by James Bongiorno, but without the Dynaguard and larger heatsinks (fan was optional on the 400 and mandatory on the 410).  It shows the mark of his excellent circuit design.  It was very conservatively specified at 0.25% distortion when the actual midband distortion (where it counts most) is below 0.03%, about ten times less.  The S/N is comparable to some of the best amplifiers ever made.

Comparing to my reference amp from the 1990's, 20 years later, the Hafler 9300 designed with MOSFETS by Jim Strickland (who also designed the Acoustats) using his patented Trans Nova circuit, what I consider to be one of the best amplifiers and amplifier designs of all time (and too often ignored), the Dynaco 410 appears to be sonically identical, except for the chassis noise of the fan and transformer.  (If you eliminate the fan, there is still notable transformer noise louder than most good amplifiers, which the fan tends to cover up.)

With two days of blind testing, the accuracy of my guessing in double blind tests between the Dynaco 410 and my reference amplifier stays around 50%.  It seems I can't actually tell them apart.

Thus, the Dynaco 410 passes, if you can hide or live with the fan noise.  In my kitchen I might not notice.